233 



tions of the shell preserved. Tlio specimens, one of which is repre- 

 sented on Plate 31, agree well with Mr. Gabb's description of N. 

 solitaria, and are almost exactly of the same dimensions as his type. 



YOLDIA ARATA. (N. iSp.) 



Plate 31, figs. 4 and 4a. 



Yoldia. {S2^. iintif.) This volume, page 72. 



Shell small, tumid, transversely elongated and very inequilateral : 

 anterior end narrowly rounded or somewhat pointed in the middle : 

 posterior end about one-third longer than the anterior, attenuated and 

 produced into a straight and narrow or slightly cm-ved and somewhat 

 broad beak. In some specimens the beaked posterior extremity nar- 

 rows gradually and almost equally both above and below, and its ex- 

 treme apex is truncated : in others the margin of the posterior end 

 rounds up obliquely from below and forms a subangular or pointed 

 junction with the shallowly concave or straight downward slope of the 

 hinge border above. Umbones small, narrow, projecting very little 

 above the hinge line and placed in advance of the middle : beaks 

 minute, curved inwards and a little downwards, with a slight inclina- 

 tion towards the posterior end. Posterior area linear-lanceolate in out- 

 line, as viewed from above, consisting of an abrupt and obtusely angu- 

 lar inflection of each valve behind the beaks. 



Surface marked with minute grooves and raised ridges which are 

 very irregular in their disposition, but which as a whole are very 

 nearly concentric. Sometimes the ridges are not continuous, and they 

 are rarely either quite pai-allel with each other or with the ventral 

 margin. In some specimens, also, the grooves are comparatively 

 broad and separated by exceedingly narrow raised lines : in others the 

 grooves and subangular ridges which alternate with them are equal in 

 breadth. 



Dimensions of an average specimen : length, ten millimetres : height, 

 six mm. and a-half : thickness through the closed valves a little over 

 foui- mm. 



South Island, Skidegate Inlet, abundant: Bear Skin Bay, Skidegate 

 Inlet, three specimens ; Cumshewa Inlet (" horizon doubtful," G. M. D.), 

 four specimens. 



